1. Field of the Invention
The present invention relates to a method for fabricating a CuInS2 thin film by metal-organic chemical vapor deposition (hereinafter, referred to as simply “MOCVD”), a CuInS2 thin film fabricated by the method and a method for fabricating an In2S3 thin film from the CuInS2 thin film. More specifically, the present invention relates to a method for fabricating a CuInS2 thin film, the method comprising fabricating a copper thin film by depositing an asymmetric copper precursor on a substrate by MOCVD and fabricating a CuInS2 thin film by depositing an indium-sulfur-containing precursor on the copper thin film by MOCVD. Furthermore, the present invention relates to a method for fabricating an In2S3 thin film for a window of a solar cell by depositing an indium-sulfur-containing precursor on the CuInS2 thin film by MOCVD.
2. Description of the Related Art
Solar cells for converting solar energy into electric energy based on semiconductor characteristics attract much attention as representatives of environmentally friendly energy sources.
Crystalline silicon solar cells have the highest market share in solar cells. Although crystalline silicon solar cells have a possibility for cost reduction by means of mass-production, they utilize bulky silicon as a raw material, thus having a limitation in reducing material costs.
Accordingly, numerous studies have recently focused on development of solar cells that satisfy requirements of low cost, high efficiency and high reliability. In particular, thin film solar cells draw a great deal of attention as leading next-generation solar cells. Thin film solar cells can be fabricated using a small amount of materials through an integrated fabrication process, thus having a possibility for a reduction in fabrication costs.
Despite such circumstances, thin film solar cells have a complicated chemical structure and necessarily involve deposition of several micrometers of a plurality of ultra-thin films. For these reasons, a commercially successful technology for thin film solar cells has not yet been developed.
CuInS2 thin films are composed of copper (Group 11, Cu), indium (Group 13, In) and sulfur (Group 16, S) in a ratio of 1:1:2. CuInS2 thin films exhibit a variety of semiconductor characteristics, thus being widely used as major materials for magneto-optical memory devices, n- or p-type semiconductors, quantum dots, solar cells, etc. In an attempt to fabricate CuInS2 thin films with low defects and superior quality, various methods have been recently reported. For example, Korean Patent Publication No. 2002-0059162 discloses a method for fabricating chalcogenide thin films with various compositions (e.g., (GeaBibSbc)Tex) via physical vapor deposition. Korean Patent No. 220,371 discloses fabrication of a CdS thin film using conventional chemical bath deposition (CBD). In addition, U.S. Pat. Nos. 6,036,822 and 5,772,431 disclose fabrication of chalcogenide thin films by electrodeposition which is a kind of electroplating method.
Further, U.S. Pat. Nos. 4,523,051, 5,045,409 and 6,444,043 disclose vacuum evaporation and close spaced sublimation. For the purpose of improvement in conventional evaporation using volatility at a high temperature, the vacuum evaporation employs simultaneous evaporation of metal compounds under vacuum to fabricate thin films at lower temperature.
The methods for fabricating thin films as mentioned above are divided into physical methods by means of phase transition such as physical vapor deposition (PVD) and vacuum evaporation, and chemical methods involving chemical reactions such as chemical bath deposition and electrodeposition.
Both the physical and chemical methods require complicated process conditions of low vapor pressure and constant solution concentration ratio to fabricate chalcogenide thin films having a specific composition. In addition, these methods have advantages of relatively low fabrication costs and simple fabrication equipment, but disadvantageously make it difficult to adjust a composition of chalcogenide to a desired level due to inherent characteristics thereof and to secure reproducibility in the thickness and uniformity of thin films.
In particular, vacuum evaporation necessarily involves deposition at a high temperature and high pressure, because chalcogen elements (e.g., S, Se and Te) are evaporated at a high temperature. Vacuum evaporation involves a complicated and troublesome procedure to deposit chalcogen elements with a desired composition due to different volatilities. In addition, vacuum evaporation has a problem of the necessity of the supplement of elements lost from deposition and subsequent thermal processing at a high temperature for phase formation [U.S. Pat. No. 6,323,417].
In addition, fabrication of thin films by chemical bath deposition is carried out by dipping a matrix in a solution and developing a thin film therein. The chemical bath deposition has general problems associated with wet processes and its application is limited to CdS thin films.
As a representative method to solve these problems, chemical vapor deposition (CVD) was suggested. Chemical vapor deposition (CVD) has advantages of uniform thin films, selective deposition, and small impurities. In addition, chemical vapor deposition (CVD) has another advantage in that thin films with a single composition can be reproductively fabricated through a simple process utilizing precursors having a constant composition. Based on these advantages, chemical vapor deposition (CVD) is widely utilized to fabricate semiconductors, e.g., metal electrodes, metal oxides, etc. However, chemical vapor deposition (CVD) requires pre-selection of metal and chalcogen elements in accordance with a specific composition, and sublimation characteristics of the precursors.
Accordingly, organometallic single precursors for chemical vapor deposition (CVD) of chalcogenide composed of constituent components with a constant composition must enable formation of a chemical complex containing a specific metal and chalcogen elements (e.g., S, Se or Te) with a constant composition, have desired volatility at a low temperature, be decomposed at a relatively low temperature, and readily form a chalcogenide phase after the decomposition due to their relatively superior thermal stability upon evaporation.
In recent years, as a result of a great deal of research on the organometallic single precursors, there was reported Group 11 copper (Cu) compound precursors comprising β-diketone or β-ketoester ligands having an alkyl group introduced in an alpha position thereof, and being coordinated to an ethylenediamine group through a Lewis base capable of imparting an unshared pair of electrons to a central copper II [Korean Patent Publication No. 1995-0009445, and U.S. Pat. Nos. 3,356,527 and 5,441,766].
In addition, there was reported organometallic single precursors comprising a chalcogen element introduced into a Group 13 metallic element, e.g., gallium (Ga), aluminum (Al) or indium (In) through a tert-butyl group [U.S. Pat. No. 5,300,320]. However, these organometallic single precursors have problems in that various metals are particularly restricted in their use to fabricate chacogenide thin films involving the necessity of various components and compositions, and an insufficient content of highly volatile sulfur (S) or selenium (Se) is used in preparation of chalcogenide composed of multimetals, as similar to vacuum evaporation.
Recently, the present inventors have reported [R2M(μ-ER1)]2 as a novel Group 13 chalcogenide precursor compound (wherein M is In and Ga; E is a chalcogen element selected from S and Se; and R1 and R2 are each independently C1-C6 alkyl) in Korean Patent Publication No. 2003-0023385. In addition, the present inventors have reported an organicmetallic compound in the form of M(E2CNR1R2)2, M′(E2CNR1R2)3 (wherein M is Zn, Cd or Hg; E is a chalcogen element selected from S and Se; and R1 and R2 are each independently C1-C6 alkyl) as a novel Group 12 or 13 chalcogenide precursor for a thin film. In accordance with this invention, the chalcogenide thin film is composed of a chacogen element and a Group 12 or 13 metal in a ratio of 1:1 and 2:3.
To fabricate high-purity CuInS2 thin films, the present inventors had earnestly researched to obtain metal chalcogenide precursors capable of avoiding the use of toxic materials (e.g., H2S), comprising Cu, In and S with a constant composition, and being chemically deposited at a relatively low temperature. As a result, the present inventors have obtained CuInS2 thin films free of impurities by developing an asymmetric copper precursor and an indium-sulfur-containing precursor, each having stability against air and heat, and sequentially depositing the precursors on a substrate by MOCVD. Furthermore, the present invention has finally completed by confirming fabrication of In2S3 thin films from the CuInS2 thin films.